Communications device and method

ABSTRACT

A communications device transmits/receives data from a mobile communications network including one or more network elements forming plural cells. Each cell is allocated a cell identifier. For each cell the network elements provide a wireless access interface providing plural communications resource elements across a frequency range of a first carrier, and divided in time into plural frames. The one or more network elements transmit in one or more of the frames a synchronization sequence, each synchronization sequence providing an indication of a cell identifier. A controller calculates an estimate of a cell identifier from the detected synchronization sequence, and uses the cell identifier to transmit/receive the data to/from the mobile communications network via the wireless access interface. A relative temporal location of the synchronization sequence within the frame provides the communications device with an indication of the cell identifier of the cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on PCT/GB2014/050863 filed Mar. 19,2014, and claims priority to British Patent Application 1305234.5, filedin the UK IPO on 21 Mar. 2013, the entire contents of each of whichbeing incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to communications devices, and methods ofcommunicating using communications devices, infrastructure equipment formobile communications networks, mobile communications networks andsystems and methods of communicating using mobile communicationsnetworks.

BACKGROUND OF THE INVENTION

Mobile communications systems continue to be developed to providewireless communications services to a greater variety of electronicdevices. In more recent years, third and fourth generation mobiletelecommunication systems, such as those based on the 3GPP defined UMTSand Long Term Evolution (LTE) architectures have been developed tosupport more sophisticated communications services to personal computingand communications devices than simple voice and messaging servicesoffered by previous generations of mobile telecommunication systems. Forexample, with the improved radio interface and enhanced data ratesprovided by LTE systems, a user may enjoy high data rate applicationssuch as mobile video streaming and mobile video conferencing that wouldpreviously only have been available via a fixed line data connection.The demand to deploy third and fourth generation networks is thereforestrong and the coverage area of these networks, i.e. geographiclocations where access to the networks is possible, is expected toincrease rapidly.

More recently it has been recognised that rather than providing highdata rate communications services to certain types of electronicsdevices, it is also desirable to provide communications services toelectronics devices that are simpler and less sophisticated. Forexample, so-called machine type communication (MTC) applications may besemi-autonomous or autonomous wireless communication devices which maycommunicate small amounts of data on a relatively infrequent basis. Someexamples include so-called smart meters which, for example, are locatedin a customer's house and periodically transmit information back to acentral MTC server data relating to the customer's consumption of autility such as gas, water, electricity and so on.

As will be appreciated a coverage area provided by a cell of a mobilecommunications network is typically limited by a distance of acommunications device from a base station and a radio environmentexperienced by the communications device. In a case in which acommunications device is disposed in an environment which is remote froma base station or is a more difficult radio reception environment, thenany improvements which can be made to a radio communications linkbetween the communications device and a base station of the networkforming the cell are desirable. This may be applicable also for examplesin which the communications devices is an MTC-type device and providedwith a low cost and therefore reduced sensitivity receiver.

SUMMARY OF THE INVENTION

Embodiments of the present invention can provide in one example acommunication device for transmitting data to and receiving data from amobile communications network. The mobile communications networkincludes one or more network elements, the one or more network elementsforming a plurality of cells of the mobile communications network, eachcell being allocated, by the mobile communications network, a differentcell identifier and for each cell the one or more network elementsprovide a wireless access interface for the communications device. Thewireless access interface provides a plurality of communicationsresource elements across a frequency range of a first carrier, anddivided in time into a plurality of frames, the one or more networkelements transmitting in one or more of the frames a synchronisationsequence being one of a set of possible synchronisation sequences, eachof the synchronisation sequences from the set providing an indication ofa cell identifier. The communications device includes a controller whichis configured in combination with the receiver unit to detect thesynchronisation sequence as being one of the predetermined set ofsynchronisation sequences, to calculate an estimate of a cell identifierof the cell using the detected synchronisation sequence, and to use thecell identifier to transmit the data to and/or receive the data from themobile communications network via the wireless access interface. Arelative temporal location of the synchronisation sequence within theframe provides the communications device with an indication of the cellidentifier of the cell, and the controller is configured to calculatethe estimate of the cell identifier based on the relative temporallocation in the frame of the synchronisation sequence in combinationwith the detected synchronisation sequence.

According to some examples a communications device can restrict a searchfor the correct cell identifier based on an identification of a relativetemporal location in which the synchronisation sequence was transmitted.In one example the synchronisation sequence itself may identify a groupof cell identifiers and the relative temporal location may identify thegroup of cell identifiers or a subset of the group and the cellidentifier may be identified using a further synchronisation sequence.

Embodiments of the present disclosure can provide an arrangement inwhich a communications device can reduce a probability of misseddetection of a cell identifier of a transmitting cell, such as forexample a physical-layer cell identifier (PCI) therefore reducing anacquisition time for a communications device to acquire the correct PCIfor a cell via which the communications device is to transmit andreceive data. Correspondingly for the same probability of correctlydetecting the cell identifier of a cell, a range of the cell in whichcommunications device is operating may be effectively extended. This isbecause in some communications systems, control and signallinginformation is encoded with the cell identifier and so thecommunications devices must detect the cell identifier in order tocommunicate via a cell of the communications network. Therefore byincreasing a probability of correctly detecting the cell identifier, therange of the cell from the base station through which the communicationsdevice is transmitting and receiving is equivalently increased. This cantherefore improve the coverage of a cell, allowing it to reach locationswhere cell acquisition signalling would be more difficult to detectsuccessfully by communications devices within the same performancerequirements specified for communications systems.

In some examples communications devices may be disposed in locationsproviding a poor radio reception environment. In some examples thecommunications devices are reduced capability devices such as MTCdevices, such as smart meters which may be installed in residentialbasements where the radio signals experience significant propagationlosses. Similarly, communications devices experiencing significantdown-link inter-cell interference may be able to acquire the cellidentifier more easily using the arrangement disclosed and so improve alikelihood of being able to correctly communicate via the mobilecommunications network. A number of failed attempts at acquiring a cellcould also be reduced, for example where the communications device canonly weakly receive synchronisation signals conveying the cellidentifier, because a probability of failure is reduced. This could tendto reduce communications device power consumption and could improvemobile device battery life.

Various further aspects and embodiments of the disclosure are providedin the appended claims, including but not limited to, an infrastructureequipment (or network element of a mobile communications network), acommunications device and method of communicating to a communicationsdevice using a mobile communications network element.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present disclosure will now be described by way ofexample only with reference to the accompanying drawings in which likeparts are provided with corresponding reference numerals and in which:

FIG. 1 provides a schematic diagram illustrating an example of aconventional mobile communications system;

FIG. 2 provides a schematic diagram illustrating an arrangement ofchannels of a wireless access interface for ten down-link sub-frames ofa conventional LTE wireless access interface;

FIG. 3 provides a schematic diagram illustrating a conventional LTEdownlink radio sub-frame;

FIG. 4 provides a schematic diagram illustrating an example of an LTEdownlink radio sub-frame in which a narrow band virtual carrier has beeninserted at the centre frequency of the host carrier, the virtualcarrier region is shown adjacent a wideband control region of the hostcarrier;

FIG. 5 provides a schematic diagram illustrating an example of a mobilecommunications system corresponding to the example shown in FIG. 1, withexample wireless access interfaces;

FIG. 6 provides a schematic representation of the wireless accessinterfaces shown in FIG. 5, showing sub-frames within a plurality offrames;

FIG. 7 provides a schematic diagram illustrating an example arrangementof synchronisation sequences of a wireless access interface for tendown-link sub-frames in accordance with the present technique;

FIG. 8 provides a schematic diagram illustrating an example arrangementof synchronisation sequences of a wireless access interface within theOFDM symbols of a sub-frame in accordance with the present technique;

FIG. 9 is a schematic block diagram of a mobile communications systemaccording to one example of the present technique;

FIG. 10 is a flow diagram illustrating the operation of a base stationin a mobile communications network according to the present technique;and

FIG. 11 is a flow diagram illustrating the operation of a communicationsdevice (UE) according to the present technique.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Example Network

FIG. 1 provides a schematic diagram illustrating the basic functionalityof a conventional mobile communications system. In FIG. 1, a mobilecommunications network includes a plurality of base stations 101connected to a core network 102. Each base station provides a coveragearea 103 (i.e. a cell) within which data can be communicated to and fromcommunications devices 104. Data is transmitted from a base station 101to a communications device 104 within a coverage area 103 via a radiodownlink. The data is transmitted from a communications device 104 to abase station 101 via a radio uplink. The core network 102 routes thedata to and from the base stations 104 and provides functions such asauthentication, mobility management, charging and so on. The basestations 101 provide a wireless access interface comprising the radiouplink and the radio downlink for the communications devices and formexamples of infrastructure equipment or network elements for the mobilecommunications network, and may be, for the example of LTE, an enhancedNode B (eNodeB or eNB).

The term communications devices will be used to refer to acommunications terminal or apparatus which can transmit or receive datavia the mobile communications network. Other terms may also be used forcommunications devices such as personal computing apparatus, remoteterminal, transceiver device or user equipment (UE) which may or may notbe mobile.

Mobile telecommunications systems such as those arranged in accordancewith the 3GPP defined Long Term Evolution (LTE) architecture use anorthogonal frequency division multiplex (OFDM) based radio accessinterface for the radio downlink (so-called OFDMA) and the radio uplink(so-called SC-FDMA). Data is transmitted on the radio uplink and on theradio downlink on a plurality of orthogonal sub-carriers. FIG. 2 shows aschematic diagram illustrating an OFDM based LTE downlink radio frame201. The LTE downlink radio frame is transmitted from an LTE basestation and lasts 10 ms. The downlink radio frame comprises tensub-frames, each sub-frame lasting 1 ms. A primary synchronisationsignal (PSS) and a secondary synchronisation signal (SSS) aretransmitted in the first and sixth sub-frames (conventionally numberedas sub-frame 0 and 5) of the LTE frame, in the case of frequencydivision duplex (FDD) system. A physical broadcast channel (PBCH) istransmitted in the first sub-frame of the LTE frame. The PSS, SSS andPBCH are discussed in more detail below.

FIG. 3 provides a schematic diagram providing a grid which illustratesthe structure of an example of a conventional downlink LTE sub-frame.The sub-frame comprises a predetermined number of symbols which aretransmitted over a 1 ms period. Each symbol comprises a predeterminednumber of orthogonal sub-carriers distributed across the bandwidth ofthe downlink radio carrier.

The example sub-frame shown in FIG. 3 comprises 14 symbols and 1200sub-carriers spaced across a 20 MHz bandwidth. The smallest unit onwhich data can be transmitted in LTE is twelve sub-carriers transmittedover one sub-frame. For clarity, in FIG. 3, each individual resourceelement is not shown, but instead each individual box in the sub-framegrid corresponds to twelve sub-carriers transmitted on one symbol.

FIG. 3 shows resource allocations for four communications devices 340,341, 342, 343. For example, the resource allocation 342 for a firstcommunications device (UE 1) extends over five blocks of twelvesub-carriers, the resource allocation 343 for a second communicationsdevice (UE2) extends over six blocks of twelve sub-carriers and so on.

Control channel data is transmitted in a control region 300 of thesub-frame comprising the first n symbols of the sub-frame where n canvary between one and three symbols for channel bandwidths of 3 MHz orgreater and where n can vary between two and four symbols for channelbandwidths of 1.4 MHz. The data transmitted in the control region 300includes data transmitted on the physical downlink control channel(PDCCH), the physical control format indicator channel (PCFICH) and thephysical HARQ indicator channel (PHICH).

The PDCCH contains control data indicating which sub-carriers on whichsymbols of the sub-frame have been allocated to specific communicationsdevices (UEs). Thus, the PDCCH data transmitted in the control region300 of the sub-frame shown in FIG. 3 would indicate that UE1 has beenallocated the first block of resources 342, that UE2 has been allocatedthe second block of resources 343, and so on. In sub-frames where it istransmitted, the PCFICH contains control data indicating the duration ofthe control region in that sub-frame (i.e. between one and four symbols)and the PHICH contains HARQ (Hybrid Automatic Request) data indicatingwhether or not previously transmitted uplink data has been successfullyreceived by the network.

In certain sub-frames, symbols in a central band 310 of the sub-frameare used for the transmission of information including the primarysynchronisation signal (PSS), the secondary synchronisation signal (SSS)and the physical broadcast channel (PBCH) mentioned above. This centralband 310 is typically 72 sub-carriers wide (corresponding to atransmission bandwidth of 1.08 MHz). The PSS and SSS are synchronisationsequences that once detected allow a communications device 104 toachieve frame synchronisation and determine the cell identity of thebase station (eNB) transmitting the downlink signal. The PBCH carriesinformation about the cell, comprising a master information block (MIB)that includes parameters that the communications devices require toaccess the cell. The data transmitted to individual communicationsdevices on the physical downlink shared channel (PDSCH) can betransmitted in the remaining blocks of communications resource elementsof the sub-frame.

FIG. 3 also shows a region of PDSCH containing system information andextending over a bandwidth of R₃₄₄. Thus in FIG. 3 the central frequencycarries control channels such as the PSS, SSS and PBCH and thereforeimplies a minimum bandwidth of a receiver of a communications device.

The number of sub-carriers in an LTE channel can vary depending on theconfiguration of the transmission network. Typically this variation isfrom 72 sub carriers contained within a 1.4 MHz channel bandwidth to1200 sub-carriers contained within a 20 MHz channel bandwidth as shownin FIG. 3. As is known in the art, subcarriers carrying data transmittedon the PDCCH, PCFICH and PHICH are typically distributed across theentire bandwidth of the sub-frame. Therefore a conventionalcommunications device must be able to receive the entire bandwidth ofthe sub-frame in order to receive and decode the control region.

Virtual Carrier

Certain classes of communications devices, such as MTC devices (e.g.semi-autonomous or autonomous wireless communication devices such assmart meters as discussed above), support communication applicationsthat are characterised by the transmission of small amounts of data atrelatively infrequent intervals and can thus be considerably lesscomplex than conventional communications devices. Communications devicesmay include a high-performance LTE receiver unit capable of receivingand processing data from an LTE downlink frame across the full carrierbandwidth. However, such receiver units can be overly complex for adevice which only needs to transmit or to receive small amounts of data.This may therefore limit the practicality of a widespread deployment ofreduced capability MTC type devices in an LTE network. It is preferableinstead to provide reduced capability devices such as MTC devices with asimpler receiver unit which is more proportionate with the amount ofdata likely to be transmitted to the device. Furthermore the receivermay be less sensitive.

In conventional mobile telecommunication networks, data is typicallytransmitted from the network to the communications devices in afrequency carrier (host frequency range) where at least part of the dataspans substantially the whole of the bandwidth of the frequency carrier.Normally a communications device cannot operate within the networkunless it can receive and decode data spanning the host frequencycarrier, i.e. a maximum system bandwidth defined by a giventelecommunication standard, and therefore the use of communicationsdevices with reduced bandwidth capability transceiver units isprecluded.

However, as disclosed in co-pending International patent applicationsnumbered PCT/GB2012/050213, PCT/GB2012/050214, PCT/GB2012/050223 andPCT/GB2012/051326, the contents of which are herein incorporated byreference, a subset of the communications resource elements comprising aconventional carrier (a “host carrier”) are defined as a “virtualcarrier”, where the host carrier has a certain bandwidth (firstfrequency range) and where the virtual carrier has a reduced bandwidthcompared to the host carrier's bandwidth. Data for reduced capabilitydevices is separately transmitted on the virtual carrier set ofcommunications resource elements. Accordingly, data transmitted on thevirtual carrier can be received and decoded using a reduced complexityor capability transceiver unit. The virtual carrier therefore provides asection of the host carrier bandwidth containing, within a restrictedbandwidth, communications resource elements which are reserved or atleast preferably allocated to reduced capability devices.

Communications devices provided with reduced complexity or capabilitytransceiver units (hereafter referred to as “reduced capabilitydevices”) could operate by using a part of its full capability (i.e.reduced capability set of its full capability) or they could beconstructed to be less complex and less expensive than conventional LTEtype devices (onwards referred to generally as communications devices).Accordingly, the deployment of such devices for MTC type applicationswithin an LTE type network can become more attractive because theprovision of the virtual carrier allows communications devices with lessexpensive and less complex transceiver units to be used.

FIG. 4 schematically represents an arbitrary downlink sub-frameaccording to the established LTE standards as discussed above into whichan instance of a virtual carrier 406 has been introduced. The sub-framecomprises a control region 400 supporting the PCFICH, PHICH and PDCCHchannels as discussed above and a PDSCH region 402 for communicatinghigher-layer data (for example user-plane data and non-physical layercontrol-plane signalling) to respective communications devices, as wellas system information, again as discussed above. The control region 400and the shared communications resources (PDSCH) 402 therefore can occupythe entire system or host carrier bandwidth. For the sake of giving aconcrete example, the frequency bandwidth (BW) of the carrier with whichthe sub-frame is associated is taken to be 20 MHz.

Also schematically shown in FIG. 4 by a shaded region 404 within theshared resources 402 is an example PDSCH downlink allocation to aconventional communications device. In accordance with the definedstandards, and as discussed above, individual communications devicesderive their specific downlink allocations 404 for a sub-frame fromPDCCH transmitted in the control region 400 of the sub-frame.

By contrast with the conventional LTE arrangement, where a subset of theavailable PDSCH resources anywhere across the full PDSCH bandwidth couldbe allocated to a communications device in any given sub-frame, in theT-shaped arrangement illustrated in FIG. 4, reduced capability devicesmaybe allocated PDSCH resources only within a pre-established reservedfrequency bandwidth 406 corresponding to a virtual carrier. Accordingly,reduced capability devices each need only buffer and process a smallfraction of the total PDSCH resources contained in the sub-frame toidentify and extract their own data from that sub-frame.

The pre-established reserved frequency bandwidth used to communicate,e.g. on PDSCH in LTE, from a base station to a communications device, isthus narrower than the overall host frequency bandwidth (carrierbandwidth) used for communicating physical-layer control information,e.g. on PDCCH in LTE. As a result, base stations 101 may be configuredto allocate downlink resources for the reduced capability device on thePDSCH 402 only within the reserved frequency bandwidth 406. As thecommunications device knows in advance that it will only be allocatedPDSCH resources within the restricted frequency band, the communicationsdevice does not need to buffer and process any PDSCH resources fromoutside the pre-determined restricted frequency band.

Example Synchronisation Sequences

As will be appreciated in accordance with known arrangements, thePSSS/SSS is provided in order for the communications devices to be ableto synchronise to the wireless access interface provided by the basestations 101 operating within a cell and also to provide a physicallayer cell identity (PCI). In LTE, a PCI is associated with each cell ofa mobile communications network. There are five hundred and four PCIs,made up of one hundred and sixty eight groups each containing threeidentities. The PSS is used to indicate the cell identity within a groupand the SSS indicates the identity of the group. An LTE network ascurrently known is planned on a cell identifier basis since the PCIswhen embedded in PSS/SSS transmissions have good de-correlationproperties allowing communications devices to detect differing cellidentities in deployments with frequency reuse factor one.

In current releases of LTE, the PSS and SSS are both transmitted overthe central sixty three subcarriers of the system bandwidth, with thed.c. subcarrier punctured. This allows a communications device to detectthe transmissions without knowing the system bandwidth. The twosequences are both transmitted in two slots per radio frame according tothe tables below, with slots and symbols numbered from zero within eachradio frame.

In FDD, the SSS is in the OFDM symbol immediately before the PSS,allowing coherent detection on the assumption that the radio channel'scoherence time is significantly longer than an OFDM symbol. In TDD, theSSS is three OFDM symbols before the PSS, meaning that coherentdetection can be used only if the radio channel's coherence time issignificantly longer than four OFDM symbols. If the communicationsdevice detects the PSS first, it can use this to estimate the channeland coherently detect the SSS. The communications device needs toblindly detect which cyclic prefix configuration is in use given theFDD/TDD mode (which might also need blind detection).

Example implementation of synchronisation sequences for the PSSs whichhave been adopted for LTE are three possible PSS, each comprisinglength-sixty three Zadoff-Chu sequences. Each SSS is a frequency-domaininterleaving of two length-thirty one secondary synchronisation codes,which themselves are two different cyclic shifts of a singlelength-thirty one M-sequence. The two secondary synchronisation codesare alternated between the two SSS transmissions in a radio frame,allowing a communications device to determine radio frame timing from asingle observation of an SSS.

Time Location Transmission of PSS/SSS

As will be appreciated a coverage area provided by a cell will belimited in dependence upon the distance of a communications device froma base station or a radio environment experienced by a communicationsdevice. It is expected in the future that a low-cost MTC type deviceswhich may be utilising a virtual carrier provided by the mobilecommunications network may be disposed in an environment where it may bedifficult to receive the PSS/SSS. Alternatively the sensitivity of areceiver of the MTC device may be lower than a conventional device inorder to reduce costs.

The conventional arrangement shown above transmits a PSS/SSS in the samesub-frame for each of the cells throughout a mobile communicationsnetwork. The present technique provides an arrangement for asynchronisation sequence to be transmitted at different temporallocations within a frame for each cell. The temporal location of thesynchronisation sequence or sequences provides an indication of a cellidentifier such as a PCI or a group of cell identifiers (PCIs) which isalso conveyed by the synchronisation sequence itself. In the examplesgiven below, which generally conform to current proposals (releases) forLTE, the synchronisation sequence comprises two parts which are theprimary synchronisation sequence (PSS) and secondary synchronisationsequence (SSS), which have been explained above with reference to FIGS.1 to 4. As explained above, embodiments of the present technique cantherefore provide in one example an arrangement for varying a positionof an additional transmission of either or both of the PSS and the SSSin a frame or a sub-frame. This arrangement will be explained in moredetail in the following section. However it will be appreciated that oneor both of the PSS and the SSS may vary in its temporal location withina frame so that the expression PSS/SSS should be interpreted as beingPSS and/or SSS.

It is known from 3GPP LTE technical document R1-072050 to provide anarrangement in which a spacing between transmissions of thesynchronisation signals is used to indicate transitions system framenumber (SFN). In one example, in the last radio frame of a super frame,the positions of the PSS/SSS in the first slot are different to in thesecond slot. A communications device can therefore determine SFNtransition by detecting the PSS/SSS.

In contrast, embodiments of the present technique arrange the symbolspacing of the PSS/SSS transmissions to convey further information tothe communications device. The further information conveyed by atime-domain placement of the PSS/SSS transmissions is relevant to thecontents of the synchronisation sequences themselves, rather than tosome other aspect of the network. Additionally, the PSS/SSS convey a PCIand/or SSS (or in lesser case PSS), rather than SFN. Finally, theproposal in R1-072050 does not propose that the symbol spacing of thePSS/SSS transmissions can be different per cell served by a basestation.

A 3GPP technical document R1-112469 proposes to use an ‘auxiliary’PSS/SSS to aid cell acquisition in a case of heavy inter-cellinterference. The auxiliary PSS/SSS are transmitted in different OFDMsymbols to the legacy sequences, and there are proposals included to addthe use of so-called ‘almost blank sub-frames’ and other resource mutingapproaches. However this represents a mere repetition of the PSS/SSS andthe temporal position of the auxiliary PSS/SSS provides no significancein this proposal in contrast to embodiments of the present technique.

Example embodiments of the present disclosure provide an arrangement inwhich the PSS transmission timing can be used to restrict the SSSdetection options for communications devices (and possibly also PCI). Byfocussing on the SSS, an advantage is provided because de-correlationproperties of M-sequences are weaker than for the Zadoff-Chu sequencesused for the PSS, so acting to reduce a probability of missed detection(P_(MD)) of the SSS, which is more desirable. Some embodiments canprovide an advantage when used with a wireless access interface formedwith an unsynchronised new carrier type (NCT) because enhancementsdescribed above can be obtained without needing more than twotransmissions of synchronisation sequences, resulting in no or at leastreduced additional overhead. The new carrier type (NCT) will beexplained in more detail below. Embodiments of the present techniquealso provide an advantage with respect to a virtual carrier (VC)deployment explained above. Consider a deployment where a VC is notbased around central resource blocks of a host carrier. A base stationcould be configured to provide a VC in which a PSS/SSS could betransmitted repeatedly within the resources of the VC rather than a hostcarrier HC, the transmission of the PSS/SSS within the VC being arrangedat a temporal location which represents the PCI conveyed by the PSS/SSSitself without affecting synchronisation of other communications devicesoperating on the host carrier. This removes a need for the VCcommunications device to access the host carrier's synchronisationsignals, thus allowing the communications device to be a strictlynarrowband device, which could significantly reduce its cost and powerconsumption.

An example illustration of a mobile communications network operating inaccordance with the present technique is shown in FIG. 5. In FIG. 5 themobile locations network includes base stations 501 which are connectedto a core network 502 and operate substantially as explained above withreference to FIG. 1. As shown in FIG. 5 each of the base stations 501serves to form a cell of the mobile communications network by arrangingfor a transmitter and receiver within the base station to transmit andreceive signals in accordance with a wireless access interface definedby a scheduler within the base station 501. Each of the base stations501 therefore provides a host carrier HC. As explained above, thewireless access interface within a host frequency bandwidth is dividedin time into frames. Each of the frames includes ten sub-frames. Thus asshown in FIG. 5 each of the host carriers HC include frames 504 whichare divided into sub-frames 506. However it has been proposed to providea further carrier for one or more cells of the mobile communicationsnetwork which may be transmitted in addition to the host carrier HC orin some examples may be instead of the host carrier HC. Thus as shown inFIG. 5 one of the base stations 501 provides a so-called new carriertype (NCT) which also provides a wireless access interface within thehost frequency bandwidth which is divided into frames 504 and sub-frames506.

It has been proposed that in contrast to a conventional host carrier, anNCT will have significantly reduced overhead compared to conventionalcarriers complying with LTE release 11 and earlier specifications. TheNCT is intended initially for deployment in a carrier aggregation (CA)scenario, where an NCT would typically be a secondary component carrier(SCC) with a legacy carrier as the primary component carrier (PCC). AnNCT may be either synchronised with the PCC or unsynchronised. In thesynchronised case, the NCT need not transmit PSS/SSS since thecommunications device will acquire synchronisation (and other timingrelated information) from the legacy PCC. In the unsynchronised case,the NCT will transmit its own synchronisation signals and thecommunications device will acquire timing information separately for thelegacy PCC and NCT SCC. Further differences on the NCT include theexpectation that cell-specific reference signals (CRS) will betransmitted: across a bandwidth potentially much smaller than the systembandwidth of a host carrier HC; on only a single antenna port (comparedto up to four ports on legacy carriers), and at a reduced, for example 5ms, periodicity (compared to every sub-frame on legacy carriers).Additionally, that in a so-called ‘standalone’ case where an NCT is notaggregated with a legacy carrier, the NCT may not transmit any legacyPDCCH, relying instead solely on EPDCCH.

As mentioned above, a cell's coverage can be limited by a distance fromthe base station 501 at which communications devices 101 are still ableto successfully detect and correctly decode the PSS and SSS to determineframe timing, SFN and PCI. A simple method to improve coverage would beto add further repetitions of PSS and/or SSS into each radio frame, inaddition to the already scheduled transmissions in the first and sixthsub-frames 0 and 5. This would tend to target improved probability ofmissed detection, P_(MD), and acquisition time primarily, but would comeat a cost of increased overhead in each radio frame, and thereforereduced user-plane capacity on PDSCH. It is therefore desirable todevelop methods for obtaining further performance improvement fromPSS/SSS in the case that they are being repeated in additionalsub-frames of a radio frame to make the most efficient use of theincreased overhead.

FIG. 6 provides a more detailed representation of the frames of the hostcarrier HC or the NCT shown in FIG. 5. As explained above there are tensub-frames within each frame. Also explained above is the breakdown ofthe resource elements for each sub-frame as illustrated for example inFIG. 3 and represented in a more simplified form in FIG. 6.

In the following description an example of a wireless access interfacewhich is arranged in accordance with FDD transmission with normal cyclicprefix (CP), but in other examples TDD may be used. In general it isassumed that the transmissions of PSS/SSS in sub-frames 0 and 5 of aradio frame, as described in current LTE specifications for FDD, willoccur as usual. In order to improve coverage and acquisition time, andby way of non-limiting example, at least one additional transmission(i.e. at least three in total) of at least one of PSS and SSS will alsooccur within the radio frame in addition to the transmission of thePSS/SSS in sub-frames 0 and 5 of a radio frame. Such additionaltransmissions would have the potential to disturb the synchronisationprocess of legacy communications devices, which are not expecting thetransmission of the additional PSS/SSS. This is particularly true of thePSS since a legacy device could detect a new PSS transmission and thenfind apparently inconsistent transmissions in the (legacy) sub-frames 0and 5. The effect of this would be implementation specific, but could ingeneral be expected to slow the cell acquisition process. A solution tothis is to generate new PSSs for use in such scenarios, e.g. by usingdifferent cyclic shifts of the Zadoff-Chu sequence that constructs them(see above). This would lose the repetition combining gain for advancedcommunications devices, but prevent disturbance to legacy devices.

As indicated above, embodiments of the present technique would beunaffected in either case. For the SSS, legacy devices with a suitableimplementation should be unaffected by additional repetitions, sincethey will acquire slot (and sub-frame) timing from the PSS and will thushave the conventional expectation regarding when the SSS should occurand could implicitly ignore any ‘unexpected’ transmissions. However, ingeneral, embodiments of the present technique could provide anarrangement in which there is only one PSS/SSS transmission within aradio frame.

FIG. 7 provides an example illustration of one embodiment of the presenttechnique. As shown in FIG. 2, and explained above, for each frame aPSS/SSS is transmitted in the same sub-frame for example sub-frame oneand sub-frame six of a frame. However a secondary or additionaltransmission of one or both of the PSS/SSS within a frame is provided inthis example in sub-frame four. The temporal location of the additionaltransmission of the second PSS/SSS is allowed to vary within the frameon a cell by cell basis. Accordingly a mobile communications device isarranged to determine the PCI of the cell, based on a relative temporalposition of the additional transmission of the PSS/SSS 700. In oneexample the temporal location of one or both of the PSS/SSS is providedby indicating a group of possible PCI values to which the PCI of thecell belongs. Arrows 702 shown in FIG. 7 illustrate that unlike thetransmission of the first PSS/SSS which always occurs in the samesub-frame for all cells in the mobile communications network, thetransmission of the second PSS/SSS may vary within the sub-frame orbetween sub-frames of the frame.

For the example of the NCT, it is possible that no first primary orsecondary synchronisation sequences are transmitted. In this example,communications devices which access communications resources of the NCTwill first gain synchronisation with the other host carrier HC, fromwhich the devices will obtain timing, and synchronisation with themobile communications network etc. However, if an NCT is notsynchronised to a legacy carrier, it will transmit its own PSS/SSS andin general it is possible that these synchronisation sequences may notbe in the same form as has been proposed for previous releases for LTE(Release-11 locations) or indeed may comprise different type ofsynchronisation sequences. A communications device which is adapted inaccordance with the present technique to transmit and receive data via amobile communications network which transmits time varyingsynchronisation sequences to assist in the acquisition of the PCI. Suchcommunications devices may also include devices which communicate via avirtual carrier VC, such as an MTC device or a device referred togenerally as a VC communications device. In contrast a “legacy”communications device is not able to operate to take advantage of thepresent technique because it is only configured to acquire the PCI for acell and synchronise to a mobile communications network using theconventional deployment of the PSS/SSS, which are in the same positionfor all cells.

PCI Subset Restriction by SS Repetition Sub-Frame Hopping

As will be appreciated from the above explanation a conventional LTEcarrier HC may be adapted to transmit a third repetition of the PSS/SSSin a sub-frame n different to 0 and 5. This third transmission may occurin any fixed OFDM symbol s of sub-frame n, but without loss ofgenerality it may be simpler for communications device implementationthat it occurs in the same OFDM symbol as the currently-specifiedrepetitions (see table above). In this embodiment, the sub-frame inwhich the third repetition occurs implies a subset of PCIs that thecommunications device is expected to search, thus reducing a probabilityof missed detection (P_(MD)) and acquisition time. To do this, thecommunications device could typically search for the presence of thethird repetition of the PSS in sub-frames other than 0 and 5 bycorrelating the possible PSS sequences with what the communicationsdevice receives in the relevant resource elements (of OFDM symbol s, inthe example) in those other sub-frames. If any of them are considered tomatch, using any suitable well-known method, then the communicationsdevice can conclude that the carrier HC transmitted by the cell providesa wireless access interface adapted in accordance with the presenttechnique.

Having reached such a conclusion, there are up to eight possiblesub-frames that could have been chosen by the base station 101, allowingthe communications device's to determine a PCI, but assisted by usingthe temporal location of the PSS/SSS to confine the search of the PCI tobe within one of eight possible subsets. One example embodiment could bethat only the SSS is in fact confined to a subset, since thecommunications device has already detected a PSS. In another example,the overall PCI is confined to a subset, determined from the temporallocation of the PSS or the SSS, allowing the communications device thepossibility of revising its original detection of even the firstestimate of the identity of the PCI carried on the PSS.

The subsets to which the SSS or PCI is confined could be:

-   -   Continuous and either ordered or in no particular order across        the whole set of SSSs or PCIs. For example if there are N        SSS/PCI values, numbered 0 . . . N with the additional        repetition detected in sub-frame n, sets of confined values        could be:        -   Ordered—n=0: {0, 1, . . . , (N/8)−1}, n=1: {N/8, . . . ,            (2N/8)−1}, . . . , n=7: {7N/8, (N−1)}. In general, for n=p:            {Np/8, (Np/8)+1, . . . , N(p+1)/8−1}.        -   Unordered—n=0: {3N/8, . . . , (4N/8)−1}, n=1: {0, 1, . . . ,            (N/8)−1}, . . . , n=7: {N/8, . . . , (2N/8)−1}.    -   Discontinuous and either ordered or in no particular order        across the whole set of SSSs or PCIs. This could mean that some        values are excluded from operation. Continuing the above        example, one discontinuous and ordered arrangement is:        -   n=0: {0, 1, . . . , (N/8)−3}, n=1: {N/8, (2N/8)−2}, . . .            n=7: {7N/8, (N−1)}, where there are some gaps, positioned            arbitrarily in this example, in permissible values.    -   Any collections of possible values, e.g. n=0: {3, 7, 99, 112, .        . . }, n=1: {34, 77, 82, . . . }, which could together cover all        or only some of the possible values.

As will be appreciated from the embodiments explained above, the mobilecommunications network may be configured to broadcast the association ofa sub-frame for the third repetition to a subset of SSSs or PCIs from acell by the base station 101 or could be defined in specifications. Ingeneral, not all sub-frames may be permitted for the third PSS/SSSrepetition to be transmitted, in which case the number of possibleconfined sets is correspondingly smaller.

In a further example embodiment, one or other of the two currenttransmissions of PSS/SSS are allowed to occur in any sub-frame, ratherthan being limited to sub-frames 0 and 5. For example, the firsttransmission could still be in sub-frame 0, but the second could beallowed to move, and the sub-frame in which the second transmission isdetected acts in the way of the third transmission described above. Thisexample avoids the increased overhead of having a third transmission,but could prevent legacy communications devices from accessing thecarrier if the second transmission were not in sub-frame 5. Therefore,this example could be more applicable to an unsynchronised NCT whichtransmits its own synchronisation sequence(s) without backwardcompatibility requirements.

For the example of a conventional host carrier in which a thirdtransmission of the PSS/SSS is allowed to vary between cells in time inaccordance with the present technique, a legacy communications devicemay not be configured to be aware of this additional transmission of thePSS/SSS which could puncture the PDSCH with no ability for the basestation to signal this to the legacy communications device, that thispuncturing of the PDSCH by the transmission of the PSS/SSS has occurred.This would tend to degrade the legacy communications device's decodingof PDSCH. Furthermore, the additional repetitions would potentiallycollide with reference signals (RS) and transmissions such as PBCH ifthey were carried in certain OFDM symbols of certain sub-frames. Inthese examples, in order for transmissions of the PDSCH or PBCH whichare expected by legacy communications devices to be unaffected by theadditional transmission of the PSS/SSS, then embodiments arrange for theadditional PSS/SSS transmissions to be prohibited from being sent insub-frames or physical resources where collisions would occur on suchOFDM symbols (which could reduce the number of sets into which PCI/SSScould be divided).

Blind Search Between Cyclic Prefix (CP) Length and FDD/TDD

In some embodiments it may be necessary for the communications device tobe able to distinguish at least between the two CP lengths, andpotentially also between FDD and TDD operation if the communicationsdevice supports both. In this embodiment, if the OFDM symbol separationis kept the same as explained above, and different between FDD and TDDthen a communications device can still distinguish between CP lengths.If the symbol separation is changed to some other fixed separation, thenit should preferably be changed to another predefined (i.e., given inspecification) value so that the communications device can more easilyconduct a blind search.

PCI Subset Restriction by Synchronisation Sequence Repetition OFDMSymbol Hopping

FIG. 8 provides an example of a sub-frame which corresponds to thediagram shown in FIG. 3 but simplified in order to illustrate exampleembodiments of the present technique. In FIG. 8 a PSS 800 and SSS 802are transmitted in resource blocks five and seven within the virtualcarrier 310 which includes a central range of frequencies 310 withinwhich the PSS/SSS are transmitted. Thus the OFDM symbol in which thePSS/SSS are transmitted can vary and the temporal location is then usedto provide an indication of the PCI as explained above. As can be seenin FIG. 8 therefore the PSS/SSS may vary in the OFDM symbol in which itis transmitted and the variation as indicated by arrows 900, 902 mayvary between cells. Thus, in this example, the third repetition of thePSS and/or SSS may vary in that the OFDM symbols in which at least oneof the PSS and SSS is located may be different in different cells toindicate a PCI value or assist in the detection of the PCI value by acommunications device. Jointly with the sub-frame n in which the thirdtransmission of the PSS/SSS is discovered, the OFDM symbol(s) chosenprovides further subsets which can be used to further reduce a rangeover which the communications device must search for a PCI of the cell,tending to further reduce P_(MD). For example consider that the basestation 101 is configured to transmit the third PSS/SSS repetition insub-frame n=p. It could be specified that the PSS will still occur inOFDM symbol 6 (in FDD) but that the SSS may occur in any other suitablesymbol s of sub-frame p, i.e. not those over which the control regionextends (if a control region exists, which may not be the case on e.g.the NCT). The communications device is expected to search all suchsymbols to determine if a SSS (from among the restricted set indicatedby n=p) has been transmitted in one of these symbols. This value stogether with the value p in this example jointly indicate the set ofconfined PCI values which the cell PCI value forms part. The improvementin P_(MD) would arise by allowing a possible misdetection of the valueof SSS in symbol s to be corrected, i.e. the communications devicecorrectly detects the presence of an SSS but has incorrectly identifiedwhich of the possible sequences the SSS is within the set of possiblesequences. By further reducing the set of possible values, some of thesemisdetections can be eliminated, thereby reducing the probability ofmis-detection P_(MD).

In some embodiments further alterations to the sub-frame contents couldbe necessary if the moveable position of (for example) the SSS collideswith reference signals (RS). Solutions include puncturing the SSS, orpuncturing the RS. The former option could be better for backwardcompatibility, particularly since the advanced communications devicewould be aware of the SSS puncturing when testing a relevant OFDMsymbol/sub-frame combination, whereas the latter option may preserve theperformance of the SSS for the advanced communications device but coulddegrade the RS performance for legacy communications devices.

In another example the transmission of an SSS may not occur in any OFDMsymbol at all in sub-frame p, and this absence can be used to create afurther set of restricted values. In other examples, the thirdrepetition of SSS can occur in any symbol of any sub-frame, i.e. notrequiring that the additional repetitions of PSS and SSS be in the samesub-frame. The sub-frame and symbol in which it occurs may then jointlyprovide a restricted set indexes in the same manner as previouslydescribed. This would clearly have a much higher search load for thecommunications device, but this would be limited by noting that the SSSneed only be searched over the restricted subset implied by theoccurrence of PSS in sub-frame n=p (as noted above). Furtherrestrictions on which sub-frame the SSS can occur in (and which thecommunications device is therefore expected to search) can be created bysimilar methods.

As will be appreciated, for the example of an unsynchronised NCT theimplementation of this example embodiment may be simpler, because it isnot necessary that three repetitions of PSS/SSS are transmitted.

As will be appreciated for the above example coherent detection of anSSS given a PSS may be possible depending on the coherence properties ofthe radio channel. Separating the PSS and SSS by more OFDM symbols maytherefore affect the applicability of coherent detection. In theparticular case of an MTC device, such as a smart utility meter, whichmay be stationary, the radio channel coherence can be expected to belong, so such devices may be tolerant of this effect.

In this embodiment, unlike the first, the OFDM symbol associationbetween PSS and SSS is evidently changed. However, if as in the example,the PSS is transmitted in a fixed OFDM symbol, and this OFDM symbol isstill different between TDD and FDD, and different between normal andextended CP then the communications device is able to distinguish.

Lower-Complexity OFDM Symbol Hopping

A computational load which must be performed by a controller in acommunications device which is searching for the SSS in accordance withthe present technique could be high because it would require thecorrelation of the received signal with all possible SSSs (although froma restricted set) in all possible OFDM symbols. Therefore in thisexample, the SSS may not be free to occur in any OFDM symbol and thecommunications device can therefore assume that it need not search someOFDM symbols. Some variations include:

-   -   Fix instead the OFDM symbol in which the SSS must occur and        apply the subset restriction on the basis of a moveable PSS.        This is likely to deliver much smaller advantages on P_(MD)        reduction since there are only three possible PSSs.    -   Specify or configure, e.g. via RRC, that if a (third repetition        of) PSS occurs in OFDM symbol s then the SSS is only allowed to        occur in a given subset of the OFDM symbols, rather than in any        of them. A scenario where this could be useful is if a        coverage-limited communications device is physically able to        receive signals from multiple eNBs but the signals are very        weak, and acquiring the cells is hard. Therefore, such a        communications device could manage to acquire a first cell, and        that eNB then RRC configures the communications device with the        OFDM symbol restrictions applicable to each other cell, thus        assisting the communications device's acquisition of those        cells, allowing it to make RRM and interference measurements,        prepare handover, etc. to those cells.    -   Specify or configure that only certain differences in OFDM        symbol number between PSS and SSS are permitted, i.e. that a        communications device finding PSS occurring in symbol Sp can        assume that SSS occurring in symbol s_(S) must obey some        restrictions on r=(s_(P)−s_(s)), such as:        -   A limit on the maximum value of |r|.        -   That r may take only a restricted set of possible values.        -   r>0 or r<0 (note that r>0 would reverse the conventional            order of PSS and SSS and could therefore have implications            for the complexity of communications device implementation).

As will be appreciated mobile communications devices adapted inaccordance with the present technique can be provided with a map of therelative position of the additional transmission of the PSS or SSS andthe PCI group or groups which are indicated by the relative temporalposition of the PSS and/or SSS. In one example this mapping istransmitted via the PBCH of the wireless access interface of the cell.

Multiple Additional Repetitions of PSS/SSS

In further examples, there can be any number of repetitions ofsynchronisation sequences within a radio frame. The joint set ofsub-frames in which the synchronisation sequences are all detected canbe used to create additional confined sets in accordance with thepresent technique. For example, consider that a third repetition ofPSS/SSS occurs in sub-frame x and a fourth in sub-frame y. Then thejoint index (x, y) replaces the index n in the examples above. Thisarrangement can be extended to further repetitions also such as a fifthrepetition in sub-frame z creating a joint index (x, y, z). In eitherexample, these additional confined subsets would allow each such subsetto be smaller even than in the first embodiment hence further reducingP_(MD).

To limit the communications device search requirements, restrictions onpossible pairings (and, in general, sets) of sub-frames could beintroduced. The kind of restrictions discussed for OFDM symbol pairingsexplained above could be used for this purpose.

Assisted GPS

In a conventional assisted GPS, the (general) cellular network providesinformation to the mobile device regarding, for example, which satelliteorbits can be received given the time-of-day and the cell's location.This reduces the search load for the mobile device, and cansignificantly reduce the so-called ‘time to first fix’. Inasmuch as thisis the network providing assistance to the communications device thereis some similarity to this disclosure, but assisted GPS should not beconsidered prior art since the assistance information regarding a firstsignal is provided by a second signal rather than being implicitlyconveyed by some inherent aspect of the first signal.

Example Mobile Communications System

FIG. 9 provides a schematic diagram showing part of an adapted LTEmobile communications system. The system includes an adapted enhancedNode B (eNB) 1001 connected to a core network 1008 which communicatesdata to a plurality of communications devices 1002 and a plurality oflegacy communications devices 1003 within a coverage area (i.e. cell)1004. Each of the legacy communications devices 1003 has a transceiverunit 1005 and a controller 1007 which is configured to detect a PSS andSSS transmitted in the sub-frames of a host carrier HC, which are thesame sub-frames which are used to transmit the PSS and SSS in othercells of the mobile communications network. The adapted communicationsdevices 1002 are configured to detect a synchronisation sequence PSS/SSSwhich varies in position within the frame from cell to cell as explainedabove. The temporal position within the frame of the PSS/SSS provides anindication of the cell identifier (PCI) which is also carried by thePSS/SSS. Therefore the adapted communications devices include acontroller 1007 which estimates the PCI using a combination of thedetected PSS/SSS and the indication of the PCI provided by the temporallocation of the PSS/SSS transmitted within the frame. Optionally thebase station 1001 transmits using a broadcast channel (PBCH) a relativemapping between the temporal location of the PSS/SSS and the PCI or thegroup of PCIs which the relative temporal position of the PSS/SSSindicates. However in other examples, the mapping between the PCI orgroup of PCIs and the temporal position of the PSS/SSS is pre-stored ina data store 1013 of the devices 1002.

The adapted eNodeB 1001 is arranged to transmit downlink data inaccordance with a wireless access interface described above for examplewith reference to FIGS. 5 to 9. A transmitter and receiver unit 1009forms the wireless access interface under the control of a controller1011, which also performs the function of an adapted scheduler toschedule the transmission of the additional PSS/SSS or varying theposition of the PSS/SSS within the frame to indicate the PCI for thecell.

The operation of a base station or eNB 501, 1001 according to thepresent technique is illustrated in one example by the flow diagram inFIG. 10, which is summarised as follows:

S2: As explained above with reference to FIG. 9 and the embodimentsexplained with reference to FIGS. 5 to 9, a base station, which may befor example an eNodeB or more generally an infrastructure equipment,which has been adapted in accordance with the present techniquetransmits and receives signals in accordance with a wireless accessinterface. The wireless access interface provides a plurality ofcommunications resource elements across a host frequency bandwidth,which are divided in time to form a plurality of frames.

S4: Optionally, in one example, the communications devices are operablein accordance with a specification providing an indication of a mappingbetween the relative displacement of synchronisation sequences which aretransmitted by the base stations and the cell identifier or the group ofcell identifiers (PCI) which are represented by each of the possibletemporal locations of the synchronisation sequences within a frame. Asexample, as indicated above the synchronisation sequence may comprisedifferent parts each part being transmitted separately. For the exampleof an LTE communications system then the synchronisation sequencecomprises a primary synchronisation sequence (PSS) and a secondarysynchronisation sequence (SSS). Thus the communications devices areprovided with an indication that the temporal position of a PSS/SSS canvary within the frame to provide an indication from their relativetemporal position of the PCI for the cell. This can be for example anindication, depending on the temporal position, of a group of which thePCI forms a member.

S6: The base station which is adapted in accordance with the presenttechnique transmits in one or more of the subframes of the wirelessaccess interface synchronisation sequences (PSS/SSS) which are each fromone of the sets of possible synchronisation sequences, each of whichidentifies one of the cell identifiers (PCI). As for the example of LTE,as explained above, the PSS provides an indication of one of threegroups of PCIs and the SSS indicates the PCI within the group. Thus byallowing the position of the SSS to vary within the frame, only threedifferent temporal locations are required to confirm that the PCIbelongs to one of three groups. Thus even if there is an error indetecting the PSS, the relative temporal location of the SSS providesthe communications device with an indication or confirmation of thegroup within of the PCI forms part.

S8: The synchronisation sequence (PSS/SSS) is transmitted in a temporallocation in the frame which provides the communications devices withinthe indication of the PCI of the cell which can then be combined withthe detected synchronisation sequence to improve the estimate of thecell identifier by the communications device.

In respective of the operations performed by a communications devicewhich is adapted in accordance with the present technique, FIG. 11provides an illustrative representation of a process performed by thecommunications device in the form of a flow diagram. The flow diagram ofFIG. 11 is summarised as follows:

S10: A communications device adapted in accordance with the presenttechnique detects a synchronisation sequence as being one of apre-determined set of synchronisation sequences transmitted by a basestation (infrastructure equipment) which has been transmitted via awireless access interface of the mobile communications network.

S12: The communications device determines a relative temporal locationof the detected synchronisation sequence within a frame of the wirelessaccess interface. The relative temporal location provides an indicationof the cell identifier (PCI) of the cell or a group of cell identifiers(PCIs) to which the cell identifier of the cell belongs.

S14: Optionally in one example there is pre-stored in a memory of thecommunications device a mapping between the relative displacement of thesynchronisation sequences within the frame and the cell identifier (PCI)or the group of cell identifiers (PCI's) represented by each of thepossible temporal locations of the synchronisation sequences. In anotherexample this mapping is received by the communications device from themobile communications network.

S 16: The communications device then calculates an estimate of the cellidentifier (PCI) based for example on a combination of the relativetemporal location in the frame of the synchronisation sequence and thevalue of the cell identifier carried by the detected synchronisationsequence itself. That is, the communications device is able to improve aprobability of correctly detecting the PCI for the cell by combining orconfirming the value of the PCI indicated by the relative temporallocation of the synchronisation sequence or using the relative temporallocation to identify a group of PCI values to which the PCI for the cellbelongs. For example, where the synchronisation sequence is comprised ofa PSS and an SSS, having detected the PCI group from the PSS, thecommunications device is able to confirm the PCI group from the relativetemporal position of the SSS. Having detected the SSS, thecommunications device is able to identify the PCI within the confirmedgroup of PCI's.

S18: The communications device then uses the cell identifier (PCI) totransmit data to and receive data from the mobile communications networkvia the wireless access interface in accordance with a conventionalarrangement. However as will be appreciated the communications devicemust detect the PCI in order to communicate data via the mobilecommunications network and in particular through the base station of thecell concerned. Thus improving the likelihood of correctly detecting thePCI value using the techniques explained above provides an advantage inreducing a likelihood of incorrectly detecting the PCI and thereforeincreases acquisition time.

Various further aspects and features of the present disclosure aredefined in the appended claims. Various combinations of the features ofthe dependent claims may be made with those of the independent claimsother than the specific combinations recited for the claim dependency.Although embodiments of the present disclosure have been described withreference to LTE, it will be appreciated that other embodiments findapplication with other wireless communication systems such as UMTS.

Embodiments refer to PSS and SSS, but the methods described are notlimited to the synchronisation sequences as currently specified in LTEand can be equally well applied to other synchronisation sequences. Suchsynchronisation sequences could be defined, for example, on the NCT orother new carriers specified in future, where the advantages ofrepetition can be further enhanced by adding techniques such as powerboosting to the new synchronisation signals.

As discussed in the embodiments explained above, there could be someincrease in the processing load at the communications device. However, apreferred implementation of such a communications device might onlybegin searching for the additional repetitions of synchronisationsequences according to the present technique once it determines that itis failing to achieve synchronisation using a conventional procedure. Inthis way, the processing load increment is only required when theadvantage it would provide is clearly required. Also note that even inLTE Release 8 systems, there is a degree of blind decoding load at thecommunications device to acquire PSS/SSS since the communications devicemust already search over FDD or TDD frame structure, and normal orextended cyclic prefix as well as the five hundred and four PCIs. Themobile communications network need not transmit the additionalrepetitions of PSS/SSS in every radio frame—an advanced communicationsdevice would obtain benefit when it does, but the data capacity of acell is higher when they are not transmitted. In a smart utility meterMTC case, the additional coverage provision of the invention might onlybe enabled at night, for example, when it could be arranged that suchMTC devices will activate since non-MTC device populations may reduce atsuch times.

REFERENCES

-   [1] R1-072050-   [2] R1-112469-   [3] PCT/GB2012/050213-   [4] PCT/GB2012/050214-   [5] PCT/GB2012/050223-   [6] PCT/GB2012/051326

The invention claimed is:
 1. A communications device for transmittingdata to or receiving data from a mobile communications network, themobile communications network including one or more network elements,the one or more network elements forming a plurality of cells of themobile communications network, each cell being allocated a cellidentifier by the mobile communications network and for each cell theone or more network elements provide a wireless access interface for thecommunications device, the communications device comprising: atransmitter configured to transmit signals representing the data to themobile communications network via the wireless access interface; areceiver configured to receive signals representing the data from themobile communications network via the wireless access interface, thewireless access interface providing a plurality of communicationsresource elements across frequency ranges of a first carrier and asecond carrier that is not synchronized to the first carrier, anddivided in time into a plurality of frames, the one or more networkelements transmitting in one or more of the frames a synchronizationsequence being one of a predetermined set of possible synchronizationsequences, each of the synchronization sequences from the set providingan indication of a cell identifier; and a controller configured to:detect the synchronization sequence as being one of the predeterminedset of synchronization sequences, determine a relative temporal locationof the synchronization sequence within the frame, calculate the cellidentifier based on the relative temporal location of thesynchronization sequence in combination with the detectedsynchronization sequence, wherein a value of the calculated cellidentifier varies for different relative temporal locations of thesynchronization sequence within the frame, and use the calculated cellidentifier to transmit the data to and/or receive the data from themobile communications network via the wireless access interface, whereinthe synchronization sequence includes a first set of synchronizationsequences and a second set of synchronization sequences, wherein thefirst set of synchronization sequences occurs in a same temporallocation of a sub-frame of the frame for all cells in the communicationsnetwork, and wherein the second set of synchronization sequences variestemporally within a sub-frame and between sub-frames of the frame in thecommunications network, wherein the first set of synchronizationsequences, which includes a first primary synchronization sequence and afirst secondary synchronization sequence, is transmitted via the firstcarrier, the second set of synchronization sequences, which includes asecond primary synchronization sequence and a second secondarysynchronization sequence, is transmitted via the second carrier, and thefirst set of synchronization sequences is transmitted in a same one ofsub-frames of the second carrier as the first carrier and a same one ofsub-frames of the first carrier for other cells of the mobilecommunications network, wherein both of the first primarysynchronization sequence and the first secondary synchronizationsequence occur in a same first temporal location of a first sub-frame ofthe sub-frames and a same second temporal location of a second sub-frameof the sub-frames for multiple frames for all cells in thecommunications network, the second temporal location being differentfrom the first temporal location, and wherein a temporal location ofboth of the second primary synchronization sequence and the secondsecondary synchronization sequence varies temporally within thesub-frame and between the sub-frames of multiple frames for differentcells in the communications network.
 2. The communications device ofclaim 1, wherein each of the frames is divided in time into a pluralityof sub-frames and the relative temporal location of the synchronizationsequence is the sub-frame in which the synchronization sequence istransmitted.
 3. The communications device of claim 1, wherein thecommunications resource elements of the wireless access interface areformed from sub-carriers of Orthogonal Frequency Division Multiplexed,OFDM, symbols, and the relative temporal location of the synchronizationsequence is the OFDM symbol within one of the sub-frames in which thesynchronization sequence is transmitted.
 4. The communications device ofclaim 1, wherein the synchronization sequence comprises a primarysynchronization sequence and a secondary synchronization sequence, thesecondary synchronization sequence being one of a set of possiblesecondary synchronization sequences, each of the secondarysynchronization sequences from the set identifying one of the pluralityof groups of cell identifiers and the primary synchronization sequenceidentifying the cell identifier within the group of cell identifiers,and wherein the controller is further configured to: detect thesecondary synchronization sequence as being one of the predetermined setof secondary synchronization sequences, detect the primarysynchronization sequence, calculate the cell identifier from acombination of the identified primary synchronization sequence and theidentified secondary synchronization sequence, wherein a relativetemporal location of at least one of the primary synchronizationsequence or the secondary synchronization sequence within the frameprovides the communications device with an indication of the group ofpossible cell identifiers that include the cell identifier, andcalculate the cell identifier based on the relative temporal location inthe frame of at least one of the primary synchronization sequence or thesecondary synchronization sequence and a combination of the detectedprimary synchronization sequence and the detected secondarysynchronization sequence.
 5. The communications device of claim 4,wherein the one or more network elements providing the plurality ofcells of the mobile communications network are arranged for each cell totransmit in each frame a first of the primary synchronization sequencesin one of the sub-frames and a first of the secondary synchronizationsequences in one of the sub-frames and to transmit a second of theprimary synchronization sequences in another of the sub-frames and asecond of the secondary synchronization sequences in another of thesub-frames, and the time of transmission of the second primarysynchronization sequence or the second secondary synchronizationsequence provides the relative temporal indication representing thegroup of cell identifiers for the cell, the first primarysynchronization sequence and the first secondary synchronizationsequence being transmitted in the same one of the sub-frames for each ofthe plurality of cells, and wherein the controller is further configuredto: detect the first primary synchronization sequence and the firstsecondary synchronization sequence, detect the second primarysynchronization sequence and the second secondary synchronizationsequence, and calculate the cell identifier of the cell based on therelative temporal location in the frame of at least one of the secondprimary synchronization sequence or the second secondary synchronizationsequence in combination with at least one of the detected first primarysynchronization sequence, the detected first secondary synchronizationsequence, the detected second primary synchronization sequence and thedetected second secondary synchronization sequence.
 6. Thecommunications device of claim 5, wherein the second carrier providescommunications resources for communications devices in addition to thefirst carrier, and the first primary synchronization sequence and thefirst secondary synchronization sequence are transmitted by the one ormore network elements in the sub-frames of the first carrier, and thesecond primary synchronization sequence and the second secondarysynchronization sequence, which are arranged to vary in the frame torepresent the group of cell identifiers of the cell are transmitted onthe second carrier of the cell, and the controller is configured tocalculate the cell identifier of the cell from a combination of therelative temporal location of at least one of the second primarysynchronization sequence and the second secondary synchronizationsequence detected from the second carrier in combination with theidentified first primary synchronization sequence and the firstsecondary synchronization sequence on the second carrier.
 7. Thecommunications device of claim 1, wherein the controller is furtherconfigured to receive from the mobile communications network anindication of a mapping between the relative temporal location of thesynchronization sequence for each cell and the group of cell identifiersor cell identifiers of the cell.
 8. The communications device of claim1, wherein the controller includes a memory that is configured to storean indication of a mapping between the relative temporal location of thesynchronization sequence for each cell and the group of cell identifiersor cell identifiers of the cell.
 9. The communications device of claim1, wherein the synchronization sequence includes a third set ofsynchronization sequences including a primary synchronization sequenceand a secondary synchronization sequence.
 10. A method of transmittingdata to or receiving data from a mobile communications network by acommunications device, the mobile communications network including oneor more network elements, the one or more network elements forming aplurality of cells of the mobile communications network, each cell beingallocated a cell identifier by the mobile communications network and foreach cell the one or more network elements provide a wireless accessinterface for the communications device, the method comprising:transmitting signals representing the data from the mobilecommunications device to the mobile communications network via thewireless access interface; receiving signals representing the data atthe mobile communications device from the mobile communications networkvia the wireless access interface, the wireless access interfaceproviding a plurality of communications resource elements acrossfrequency ranges of a first carrier and a second carrier that is notsynchronized to the first carrier, and divided in time into a pluralityof frames, the one or more network elements transmitting in one or moreof the frames a synchronization sequence being one of a set of possiblesynchronization sequences, each of the synchronization sequences fromthe set providing an indication of a cell identifier of the cell;detecting the synchronization sequence as being one of the predeterminedset of synchronization sequences; determining a relative temporallocation of the synchronization sequence within the frame; calculatingthe cell identifier based on the relative temporal location of thesynchronization sequence in combination with the detectedsynchronization sequence, wherein a value of the calculated cellidentifier varies for different relative temporal locations of thesynchronization sequence within the frame; and using the calculated cellidentifier, by the communications device, to transmit the data to and/orreceive the data from the mobile communications network via the wirelessaccess interface, wherein the synchronization sequence includes a firstset of synchronization sequences and a second set of synchronizationsequences, wherein the first set of synchronization sequences occurs ina same temporal location of a sub-frame of the frame for all cells inthe communications network, and wherein the second set ofsynchronization sequences varies temporally within a sub-frame andbetween sub-frames of the frame in the communications network, whereinthe first set of synchronization sequences, which includes a firstprimary synchronization sequence and a first secondary synchronizationsequence, is transmitted via the first carrier, the second set ofsynchronization sequences, which includes a second primarysynchronization sequence and a second secondary synchronizationsequence, is transmitted via the second carrier, and the first set ofsynchronization sequences is transmitted in a same one of sub-frames ofthe second carrier as the first carrier and a same one of sub-frames ofthe first carrier for other cells of the mobile communications network,wherein both of the first primary synchronization sequence and the firstsecondary synchronization sequence occur in a same first temporallocation of a first sub-frame of the sub-frames and a same secondtemporal location of a second sub-frame of the sub-frames for multipleframes for all cells in the communications network, the second temporallocation being different from the first temporal location, and wherein atemporal location of both of the second primary synchronization sequenceand the second secondary synchronization sequence varies temporallywithin the sub-frame and between the sub-frames of multiple frames fordifferent cells in the communications network.
 11. The method of claim10, wherein each of the frames is divided in time into a plurality ofsub-frames and the relative temporal location of the synchronizationsequence is the sub-frame in which the synchronization sequence istransmitted.
 12. The method of claim 10, wherein the communicationsresource elements of the wireless access interface are formed fromsub-carriers of Orthogonal Frequency Division Multiplexed, OFDM,symbols, and the relative temporal location of the synchronizationsequence is the OFDM symbol within one of the sub-frames in which thesynchronization sequence is transmitted.
 13. The method of claim 10,wherein the synchronization sequence comprises a primary synchronizationsequence and a secondary synchronization sequence, the secondarysynchronization sequence being one of a set of possible secondarysynchronization sequences, each of the secondary synchronizationsequences from the set identifying one of the plurality of groups ofcell identifiers and the primary synchronization sequence identifyingthe cell identifier within the group of cell identifiers, the methodfurther comprising: detecting the secondary synchronization sequence asbeing one of the predetermined set of secondary synchronizationsequences; and detecting the primary synchronization sequence;calculating the cell identifier of the cell from the detectedsynchronization sequence; calculating the cell identifier from acombination of the detected primary synchronization sequence and thedetected secondary synchronization sequence, wherein a relative temporallocation of at least one of the primary synchronization sequence or thesecondary synchronization sequence within the frame provides thecommunications device with an indication of the group of possible cellidentifiers of which the cell identifier that include the cellidentifier; and calculating the cell identifier based on the relativetemporal location in the frame of at least one of the primarysynchronization sequence or the secondary synchronization sequence incombination with at least one of the detected primary synchronizationsequence and the detected secondary synchronization sequence.
 14. Themethod of claim 13, wherein the one or more network elements providingthe plurality of cells of the mobile communications network are arrangedfor each cell to transmit in each frame a first of the primarysynchronization sequences in one of the sub-frames and a first of thesecondary synchronization sequences in one of the sub-frames and totransmit a second of the primary synchronization sequence in another ofthe sub-frames and a second of the secondary synchronization sequencesin another of the sub-frames, and the time of transmission of the secondprimary synchronization sequence or the second secondary synchronizationsequence provides the relative temporal indication representing thegroup of cell identifiers of the cell, the first primary synchronizationsequence and the first secondary synchronization sequence beingtransmitted in the same sub-frames for each of the plurality of cells,the method further comprising: calculating the cell identifier of thecell based on the relative temporal location in the frame of at leastone of the second primary synchronization sequence or the secondsecondary synchronization sequence in combination with at least one ofthe detected first primary synchronization sequence, the detected firstsecondary synchronization sequence, the detected second primarysynchronization sequence and the detected second secondarysynchronization sequence.
 15. The method of claim 14, wherein the secondcarrier provides communications resources for communications devices inaddition to the first carrier, and the first primary synchronizationsequence and the first secondary synchronization sequence aretransmitted by the one or more network elements in the one of thesub-frames of the first carrier, and the second primary synchronizationsequence and the second secondary synchronization sequence, which arearranged to vary in the frame are transmitted on the second carrier ofthe cell, and the calculating the cell identifier comprises calculatingthe cell identifier from a combination of the relative temporal locationof the second primary synchronization sequence and the second secondarysynchronization sequence detected from the second carrier with thedetected first primary synchronization sequence and the detected firstsecondary synchronization sequence on the second carrier.
 16. The methodof claim 10, further comprising receiving from the mobile communicationsnetwork an indication of a mapping between the relative temporallocation of the synchronization sequence for each cell and the cellidentifier or the group of cell identifiers of the cell.
 17. The methodof claim 10, further comprising storing an indication of a mappingbetween the relative temporal location of the synchronization sequencefor each cell and the group of cell identifiers or cell identifiers ofthe cell.